Micro-sized needles, i.e. needles having diameters and lengths in the micrometer range, find great utility i.a. in many medical applications. As an example, injection of liquid such as drugs of various kinds, vaccines etc. through the skin is often performed. However, when ordinary syringes are used for the administration it can sometimes cause pain or discomfort to a patient. Micro-needles can be used in such applications to reduce the pain experienced by the patient. In particular arrays of hollow micro-needles provided on a patch that is applied to the skin and through which medicaments are administered will reduce the pain and discomfort substantially.
Other fields of application are various electrical measurements performed on the skin, using arrays of micro-needles.
There are several patents granted for methods of making micro needles, a few of which are recited below.
US published patent application 2004/0054393 (corresponding to EP 1 164 928) (Stemme et al) discloses a medical electrode for obtaining biopotentials from the skin of a subject or electrically stimulating the subject's skin and deeper tissue layers. The electrode has a carrier base member from which project a plurality of spikes arranged in an array on one surface of the base member. The spikes are sufficiently long to penetrate through the stratum corneum into the stratum germinativum of the subject's skin. The spikes may be formed by a deep reactive ion etching process on a silicon wafer forming the base member. A fluid container may be formed on another surface of the skin for providing a drug to the surface of the skin through holes in the base member. The action of the spikes on the skin enhances administration of the drug.
Boisen et al in a paper entitled “Novel AFM Probes with directly fabricated tips”, J. Micromechanical Microeng., 6(1):58, 1996, disclose solid micro-needles and methods of making such. In particular the needles have what is referred to as “rocket tips”, and are made by an isotropic-anisotropic-isotropic etch sequence.
U.S. Pat. No. 5,855,801 (Lin et al) discloses a method of fabricating a microstructure. The method includes providing a substrate for forming an interface region and an elongated portion extending away from the interface region. A patterned, non-planar etchable structure is formed on one side of the elongated portion of the substrate. An unetchable membrane layer is deposited atop the etchable structure. At least one etching hole is formed in the membrane layer. The etchable structure is etched by placing an etchant into the etching hole to form a cavity underneath the membrane layer, thereby producing a shaft.
WO 2003/015860 (Stemme et al) discloses a method of making a needle having side openings. It comprises providing a mask on the front side of an etchable wafer such that the vertical projection of said mask at least partially covers the extension of a hole made in the back side. Said mask is isotropically underetched to remove wafer material. An anisotropic etch forms a protruding structure. Optionally a second isotropic etch on said protruding structure exposes the blind hole. Optionally a final anisotropic etch extends the needle without forming side openings. The position and extension of the mask relative the position and dimension of the hole is such that said side openings form during either said anisotropic etc or said second isotropic etch.
U.S. Pat. No. 6,334,856 (Allen et al) discloses a method for making a microneedle. It includes forming a micromold having sidewalls which define the outer surface of the microneedle, electroplating the sidewalls to form the hollow microneedle, and then removing the micromold from the microneedle.
U.S. Pat. No. 6,533,949 (Yeshurun et al) discloses a method for processing a wafer to form a plurality of hollow microneedles projecting from a substrate includes forming, by use of a dry etching process, a number of groups of recessed features, each including at least one slot deployed to form an open shape having an included area and at least one hole located within the included area. The internal surfaces of the holes and the slots are then coated with a protective layer. An anisotropic wet etching process is then performed in such a manner as to remove material from outside the included areas while leaving a projecting feature within each of the included areas. The protective layer is then removed to reveal the microneedles.
A general review on micro needles is found on the internet at the site: http://www.mne.umd.edu/
A problem with prior art devices having micro-needles used in the medical field is to achieve appropriate penetration of the outer skin layer, the stratum corneum. In particular it is important to avoid deep penetration so as to intervene with the nerves.